1. Field of the Invention
The present invention relates to a semiconductor device which transmits and receives data by wireless communication and particularly relates to a semiconductor device which uses an amplitude modulation method for data transmission.
2. Description of the Related Art
In recent years, a small semiconductor device in which a small IC chip and an antenna for wireless communication are combined (hereinafter also referred to as a semiconductor device or an RF chip; and also called an RFID tag, a wireless tag, an ID tag, an IC tag, or an RF tag) has attracted attention. The semiconductor device can transmit and receive data without contact, for example, write data or read data, by transmission and reception of a radio signal (operating magnetic field) with use of a wireless communication device (hereinafter referred to as a reader/writer).
As an application field of a semiconductor device which transmits and receives data by wireless communication, product management in the distribution industry is given as an example. Product management using a bar code and the like is the mainstream at present; however, since bar codes are read optically, data cannot be read in some cases when there is an interrupting object. On the other hand, when data is transmitted and received without contact with use of a wireless communication device, data of the semiconductor device is read wirelessly; thereby, data can be read even when there is an interrupting object. Thus, improvement in efficiency, cost reduction, and the like of product management are expected. Further, wide range of applications including boarding passes, airplane tickets, and automatic payment of fares are expected (e.g., see Reference 1: Japanese Published Patent Application No. 2005-346622). A system such that people and objects are managed and identified by a small semiconductor device which transmits and receives data by wireless communication is called RFID (Radio Frequency IDentification), and has attracted attention as fundamental technology of the IT society.
As standards of communication using RFIDs, ISO/IEC 15693 and the like are established, for example. According to ISO/IEC 15693, a frequency of 13.56 MHz±7 kHz is used for a carrier wave in a radio signal, and an ASK (Amplitude Shift Keying) method is used for transmitting and receiving data between a reader/writer and a semiconductor device.
FIG. 2 shows an example of a radio signal when data is transmitted and received between a reader/writer and a semiconductor device in the ASK method. In FIG. 2, a radio signal 201 is an electromagnetic wave which oscillates at the frequency of a carrier wave. Data transmitted from the radio signal 201 is represented by an envelope 202 of the amplitude of the radio signal 201. Hereinafter, the maximum amplitude of the radio signal 201 is represented as “W”, and the minimum amplitude thereof is represented as “N” for simplification. For example, when the semiconductor device receives data transmitted from the reader/writer, a value of the data is identified in accordance with a predetermined encoding method depending on a state of occurrence of “N” in the radio signal 201. Specifically, the value of the data can be identified in accordance with a predetermined encoding method depending on timing, a period, the number of times, and the like of the occurrence of “N”. Further, when the semiconductor device transmits data to the reader/writer, the radio signal 201 is modulated in accordance with a predetermined encoding method depending on the value of the data, and a state of occurrence of “N”, specifically, timing, a period, the number of times, and the like of the occurrence of “N” can be changed. As an encoding method, a “one out of four” method established by ISO/IEC 15693 can be used, for example.
Note that data which is transmitted from the reader/writer to the semiconductor device includes a reset signal showing reset of a circuit in the semiconductor device, a SOF (Start Of Frame)/EOF (End Of Frame) signal showing start/end of data transmission from the reader/writer to the semiconductor device, a data signal to be transmitted to the semiconductor device, a synchronizing signal for synchronizing clock signals in the semiconductor device, a parity signal used for checking whether a data transmission error from the reader/writer to the semiconductor device occurs, and the like. Further, data which is transmitted from the semiconductor device to the reader/writer includes a SOF (Start Of Frame)/EOF (End Of Frame) signal showing start/end of data transmission from the semiconductor device to the reader/writer, a data signal to be transmitted to the reader/writer, and the like.
In such a data transmission method, the amplitude of the radio signal is changed by data transmitted from the semiconductor device to the reader/writer, even when data is not transmitted from the reader/writer to the semiconductor device. Therefore, in some cases, the semiconductor device mistakes data transmitted from the semiconductor device itself for data transmitted from the reader/writer.